What is the best formula milk you've given your baby so far?

My child just had her last bottle with Promil powder (Promil Kid for 1-3 years old) and she will be shifting to Nido Jr. (Nestle) because she is still constipated with Promil. I have tried Alactamil, S-26, lactose free Promil and S-26 but it didn't make her digestion and bowel movement any better.
My eldest used Mylac, Promil and now, Nido or Bear brand depending on the budget. What formula milk did you give you child and what was the effect? What are your considerations in choosing a formula milk aside from Doctor's prescription?